Oxidative pressure leaching is a well known technique for extracting nonferrous metal values from sulfidic materials. The technique has been applied to ores, concentrates and mattes. The present process may be used for treating mattes containing only copper and nickel or cobalt, but it is particularly applicable for the treatment of copper-nickel-cobalt-iron mattes.
These mattes are sulfidic intermediate products obtained by pyrometallurgical techniques, e.g. by smelting and converting of various mineral sulfide-containing ores or concentrates. They can also be obtained by processing of oxidic raw materials, for instance, ocean floor nodules containing the nonferrous metals as described by R. Sridhar et al, in the JOURNAL OF METALS, pp. 32-37 (April 1976). They further can be obtained by pyrometallurgical processing of various residues, e.g. residue from carbonylation process, and intermediates, e.g. slags high in Co, Cu and Ni such as finishing converter slags, etc. Some metallurgical intermediates, e.g. those from copper-nickel converter matte separation processes, can be regarded as products similar to mattes. Scrap can also be used as a source for obtaining mattes.
The smelting of the crude ores, nodules, concentrates, or residues and metallurgical intermediates, is primarily carried out to separate the valuable nonferrous metals from the gangue in order to obtain an intermediate product matte more concentrated in the nonferrous metals. The gangue is discarded as a slag. Not all of the iron will be slagged as it is known that iron in the matte would minimize losses of the valuable metals, particularly cobalt, to the slag. The primary product matte is then converted to eliminate the iron as an iron silicate slag.
The final iron content of a cobalt-containing matte after converting is always decided on economic grounds and usually maintained at a level of about 3%. Although higher iron content of the matte could give better cobalt extraction, the 3% iron content level is regarded in most cases as the optimum because higher cobalt extraction at higher iron level may not cover the cost of additional iron removal in subsequent operations of separating and refining the nonferrous metals. This invention, contrary to the conventional operations, provides new possibilities for better cobalt recovery. Since the iron can be removed economically at a later stage, a much higher iron content in the matte can be tolerated and, consequently, cobalt losses to the iron silicate slag can be significantly decreased. In general, mattes containing up to as high as 10-15% of iron and even higher can be processed in accordance with the present invention, thereby providing higher cobalt extraction without excessive expense for iron rejection while separating and refining the nonferrous metals.
The aforementioned ores, concentrates, residues, intermediates, slags, scrap, etc., containing nonferrous metals, as well as iron and possibly precious metals, are usually processed in one way or another with the purpose of extracting the metal values into mattes which, in turn, can be subjected to further treatment according to the present invention. Copper-nickel-cobalt-iron mattes may have a wide composition range. In general, they contain varying amounts of copper, nickel, cobalt, iron and sulfur. In addition they may contain precious metals and some impurities, for example, As, Sb, Pb, etc.
The present invention is not limited either to any specific composition of the matte or to any specific pyrometallurgical technique of producing the same. In general, mattes which can be processed according to the present invention may contain approximately: 20 to 60% copper, up to 15-60% nickel, up to 20-50% cobalt, up to 20-30% iron, up to 20-25% sulfur, provided either nickel or cobalt or both are present.
It is usually assumed that, at high temperatures of pyrometallurgical processes, Cu.sub.2 S, Ni.sub.3 S.sub.2, Co.sub.9 S.sub.8 and FeS are the stable sulfides of the metals thus showing that, in general, mattes are deficient in sulfur to convert all the metals into their sulfates. In fact, mattes are even more sulfur deficient relative to their molar metal contents than it follows from the above sulfide formulae. Usually when such mattes are cooled, metallic particles are precipitated in addition to crystallized sulfides.
The term "selectivity" as applied to metallurgical technology has been used to signify that at least one chemical element is separated from at least another one or from the bulk of the material. For example, iron is separated from the nonferrous metals into an iron silicate slag in the course of converting, or copper is separated from nickel and/or cobalt by one way or another, etc.
Many methods of oxidative pressure leaching of sulfidic materials containing copper, nickel, cobalt and iron are known for dissolving the nonferrous metals under oxygen pressure in acidic or ammoniacal aqueous media. Processes of this type are described in U.S. Pat. Nos. 2,746,859; 3,174,849; 3,642,435; 3,975,190, and in an article by R. F. Pearce et al in the JOURNAL OF METALS, January 1960, pp. 28-32, etc. Some of these processes may provide selective dissolution of all the nonferrous metals from iron and precious metals, but in general, they do not separate the nonferrous metals themselves. A significant economic advantage, however, may be achieved if in the course of oxidative pressure leaching itself the separation of copper from nickel and/or cobalt is realized.
In recent years processes using selective leaching to separate copper from nickel and/or cobalt have been reported. Among the processes of this type are the process presented by R. P. Plasket et al at the 103rd Annual Meeting of the AIME, Dallas, Tex., Feb. 24-28, 1974 and a process described by I. N. Maslenitsky et al in PROGRESS IN EXTRACTIVE METALLURGY, Vol. I, pp. 121-126, 1973. Both these processes require at least two oxidative pressure leach stages. In the Plasket et al process both stages of oxidative pressure leach are carried out under highly corrosive conditions of relatively low pH and temperature above 130.degree. C. Thereafter two additional operations are required for the removal of copper and iron. In the latter process, in addition to the shortcoming of two stage autoclave leaching, this process requires a commercially unacceptable critical limitation to 10-20 minutes in the first stage leach, otherwise the copper content of the nickel-cobalt solution becomes unacceptably high and the whole process, consisting of the two mutually interdependent stages, becomes inoperable and impossible to control.
A process for selective recovery of nickel from a nickel-copper matte consisting essentially of nickel, copper, sulfur and minor amounts of iron and precious metals--with only one stage of oxidative pressure leaching--is described in U.S. Pat. No. 3,652,265. After adjusting the sulfur content of a matte water slurry with elemental sulfur to obtain the nickel to sulfur molar ratio of 1:1, the sulfur adjusted slurry is subjected to the oxidative pressure leach, typically, at the temperature of 165.degree.-180.degree. C. under oxygen partial pressure of about 7-10 kg/cm.sup.2 for 10-22 hours, to convert all the sulfur and the metallic values to sulfate and oxyhydrates, respectively. The overall process requires a long time and relatively high temperatures for the oxidative leach, and the leach itself does not result in sufficient separation of nickel and copper. To achieve the desired nickel recovery and copper to nickel ratio in the residue, lengthy equilibration steps are needed.
The treatment of matte containing 40.0% Ni, 39.5% Cu, 16.4% S and 0.2% Fe is reported by Z. R. Llanos et al in CIM Bulletin, pp. 74-81 (February 1974). In this process the matte is first subjected to an aerated atmospheric leach with recycle electrolyte from the copper tank house, and the leach residue is then subjected to autoclave oxidative leaches followed by atmospheric digestion of the obtained slurries with H.sub.2 SO.sub.4 additions resulting in the feed solution for copper electrowinning. The overall process has a number of disadvantages, of which the most important ones are: low direct extraction (.about.50%) of nickel into the nickel pregnant solution and recirculation of the residual nickel through the cooper electrowinning operation; large recycle of the copper from electrowinning through the nickel atmospheric leaching and further through the autoclave leaching and atmospheric digestion where the recycled copper is firstly precipitated and then dissolved all over again; and complexity of the overall process. Also, this process is reported to require high temperature and total pressure, viz. 180.degree.-200.degree. C. and 42 kg/cm.sup.2 in the autoclaves.
It is an object of the present invention to treat copper-nickel-cobalt-iron mattes to extract selectively the nonferrous metal values. It is another object to achieve such selective extraction using only one stage of oxidative pressure leaching. It is a further object to provide an overall economically feasible process for obtaining a higher degree of extraction efficiency for nickel and, especially, for cobalt values. Still another object is to isolate nickel and cobalt values in a solution which is essentially free of iron, thereby eliminating the requirement for a separate iron removal step. An additional object is to provide a process for nickel and cobalt isolation into a solution which is essentially free of copper and iron without using any external reagents. A further object is to provide a method for separating copper into an oxidative pressure leach residue from which it can be conveniently and rapidly solubilized in industrially and economically suitable solvents at ambient conditions. Another object is to minimize nickel and cobalt content of the copper-bearing residue and, thereby, minimizing recirculation of nickel and cobalt through copper-extracting operations. A further object is to eliminate copper recycling through nickel and cobalt-extracting operations and multiple precipitation and subsequent dissolution of the copper. An additional object is to provide an oxidative pressure leach process for the extraction of nonferrous metal values in which the pH level in the autoclave is not lower than 3.0-3.5 and the temperature is not higher than 130.degree. C., thereby enabling the use of much less expensive equipment. And a further object is to provide a simple and operable process with a minimum of operational steps and maximum operational stability.
These and other objects are accomplished in the process of the present invention as will be appreciated by reference to the description and examples given below and to the accompanying drawings.